.img{
    width: 350px;
    height: 350px;
    background-image: url(../imgs/1.jpg);
    float: left;
    position: relative;
}
.enlarge{
    width: 200px;
    height: 200px;
    background-image: url(../imgs/1.big.jpg);
    display: none;
    float: left;
    background-repeat: no-repeat;
}
.img .mask{
    position: absolute;
    top: 0;
    left: 0;
    width: 100px;
    height: 100px;
    background-color: orange;
    opacity: 0.5;
    display: none;
}
.box{
    width: 600px;
    height: 350px;
}